The capture of energy from a stimulus and the initiation of a response signal in the nervous system is termed sensory transduction. In the chemosensory pathways, including taste and smell, chemical stimulant molecules often act by binding to highly specific receptor sites on the membrane of the sensory cell. This binding is in some way coupled to a characteristic change in the membrane that initiates an electrical signal. It is likely that in many cases the transduction process involves internal messenger molecules or ions, such as cyclic nucleotides,calcium, or lipid metabolites. The microscopic aquatic single-celled organism Paramecium shows simple chemosensory behavior, has many interesting mutant strains, and its cell membrane has several properties similar to excitable nerve cells. It has been a very successful model system for clarifying some aspects of chemosensory reception. This project will explore the structure, function, and distribution of chemoreceptor sites in Paramecium, and how they interact with levels of internal calcium in the transduction process. One of the important stimulus molecules is a cyclic nucleotide compound, cAMP, closely related to the well-known ATP that has vital metabolic functions. Molecular genetics will be used to clone and sequence the gene for the membrane protein thatacts as the binding receptor site for cAMP. Once the gene sequence is predicted, antibodies to shorter peptide sequences within that gene will be made to probe receptor structure and distribution, and to predict the functional domains of the receptor molecule. Calcium levels will be measured using dyes, and normal and mutant strains will be used to characterize the nature of the "molecular pump" driven by ATP that extrudes calcium. Findings from this novel model system will have considerable impact on chemosensory work, because very few chemosensory receptors have yet been characterized, and no common patterns have emerged yet. The results, along with those on the calcium pump, are likely to be important also to neuroscientists in general, since many of the mechanisms may be similar to those found in the functioning synaptic contacts between neurons.
